1. Field of the Invention
The present invention relates to a process for producing aliphatic polyesters having at least 50% of aliphatic hydroxycarboxylic acid units, which are biodegradable polymers usable as medical materials and as substitutes for general-purpose resins.
More precisely, the invention relates to a process for producing high-molecular aliphatic polyesters having at least 50% of aliphatic hydroxycarboxylic acid units, which comprises solid-phase polymerization of a crystallized, low-molecular aliphatic polyester prepolymer having at least 50% of aliphatic hydroxycarboxylic acid units in the presence of a catalyst.
The invention includes a process for preparing high-molecular aliphatic polyesters with the amount of the catalyst remaining therein being reduced, which comprises, for example, polymerizing an aliphatic hydroxycarboxylic acid in the presence or absence of a specific volatile catalyst and in the presence or absence of a solvent to give an aliphatic hydroxycarboxylic acid prepolymer having a relatively low molecular weight, then effectively crystallizing the resulting aliphatic hydroxycarboxylic acid prepolymer, and thereafter polymerizing it in a solid phase in the presence of a specific volatile catalyst at a temperature not higher than the melting point of the prepolymer.
2. Description of the Related Art
Associated with the protection of the environment, waste disposal is a problem in these days. In particular, wastes of general-purpose polymer moldings and processed goods are not biodegradable and are therefore problematic in that, when buried for land reclamation, they will remain semi-permanently in the land as impurities, because they are lacking in ability to degrade or disintegrate by the action of microorganisms. And, in addition, additives such as plasticizer and others will be released out from them to pollute the environment.
Further, when the wastes are incinerated, the combustion heat from them is high and will damage furnaces. In addition, the waste incineration brings about other serious problems in that the exhaust fumes and gas will cause air pollution, ozone layer destruction, global warming, acid rain, etc.
In that situation, there is increasing a great demand for polymer materials which are tough enough for general-purpose applications and of which the wastes are, when buried for land reclamation, degradable to be harmless, and, even when incinerated, produce low-temperature combustion heat that will not damage furnaces. At present, however, satisfactory polymer materials that could satisfy the demand are not available.
Polylactic acid which is one of aliphatic polyhydroxycarboxylic acids is highly transparent and tough and is characterized in that it is easily hydrolyzed in the presence of water. Therefore, when used as a general-purpose resin, it is gentle to the environment, since its wastes are easily degraded to be harmless, not polluting the environment. When used as a medical material to be left in bodies, it is degraded and absorbed in bodies, after having accomplished its object, and is therefore not toxic to bodies but is gentle thereto. The excellent properties of polylactic acid have already been noticed prior to the present application.
It has heretofore been known that high-molecular aliphatic polyhydroxycarboxylic acids are prepared, for example, according to a method of ring-opening polymerization of cyclic dimers of aliphatic hydroxycarboxylic acids, such as glycolides, lactides and others, or a method of direct dehydrating polycondensation of aliphatic hydroxycarboxylic acids such as lactic acid, glycolic acid and others (see U.S. Pat. No. 5,310,865). The ring-opening polymerization of cyclic dimers of aliphatic hydroxycarboxylic acids is generally carried out while the dimers are melted; and the direct dehydrating polycondensation of aliphatic hydroxycarboxylic acids is generally carried out in an organic solvent. The ring-opening polymerization method requires expensive lactides and the like that shall be prepared in a complicated process; and the direct dehydrating polycondensation method must be conducted in an organic solvent and the volume efficiency of the reaction therein is low.
JP-A 5-255488 (EP-A-500098) discloses a technique of heating a powdery or granular, low-molecular homopolymer or copolymer of L- and/or D-lactic acid having a degree of crystallinity as measured through X-ray diffractometry of at least 10%, in vacuum and/or in an inert gas atmosphere at a temperature higher than the glass transition temperature of the polymer but lower than the melting point thereof to thereby increase the molecular weight of the polymer. The invention disclosed in that JP-A or EP-A is characterized in that the polymerization is carried out in the absence of a catalyst to give a polymer with no catalyst residue. They say that the polymer produced therein is highly safe and is important for the technique of controlled release of medicines in bodies and for the production of bioabsorbable artificial organs.
However, as not employing a catalyst, high-molecular polymers capable of being substitutes for general-purpose resins that are required to have high toughness could not be produced in the disclosed invention.